The present invention concerns a fixed spike or a sleeve-mounted spike, fitted on a vehicle tire.
Increasing traffic load and spike tires in combination have proved to be a remarkable road attrition factor. In some countries this has even led to prohibition of spike tires, or at least to considerable restrictions.
In Nordic countries, the beneficial effect of anti-slip means on the safety and flexibility of traffic has on the other hand been irrefutably demonstrated, and this effect should not be sacrificed: instead, the drawbacks should merely be eliminated. Good results will be achieved by further developing both the road super-structures and the anti-slip tires.
As a pneumatic tire rolls on an even surface, it is considerably flattened radially, owing to its flexibility, whereby in the contact region longitudinal as well as transverse forces are generated owing to changes of the rolling radius.
When the tire rolls and the spike meets the road surface, a very rapid impact against the road is produced owing to the kinetic energy of the rubber and the visco-elastic nature of the rubber entailing resistance to fast movements.
When a spike approaches the point of contact with the road, the tire body undergoes bending such that the radius of the bent part is significantly smaller than that of equivalent parts of the load-free tire. This deflects the spike, which has been mounted at right angles against the surface, to assume a vertical position before contact with the road. Owing to the protrusion of the spike tip, however, the spike is not turned into sufficiently upright position; it meets the road surface in an oblique position. At this stage, the forces due to slippage tendency also begin to exert their influence.
When the rotating tire and the piece projecting therefrom (the spike) met the road surface, an impact effect acting on the road is produced by the kinetic energy and the rapid deformation of the rubber. Road attrition can be significantly reduced if this impact effect can be eliminated or reduced. With the aid of the present invention the impact due to the visco-elasticity of the rubber is reduced.
The phenomenon counteracting the rapid impact-type deformations of the rubber can be represented with a model consisting of an elastic spring and a viscous liquid cylinder or of combinations of these. The principle is that the viscous component becomes predominant in the case of rapid deformations, which is just the type that is concerned at the initial moment of the tire's road contact. The visco-elastic impact is described by a mathematical model worked out by Floor: ##EQU1## where
.SIGMA.=Deformation
.sigma.=Stress
E=Modulus of elasticity
t=Time
.lambda.=Retardation time
It is apparent from the form of the mathematical model that when t, or the deformation time, is very short or becomes shorter, the deformation required for producing a given stress state decreases exponentially. This is due to the fact that when the viscous component of the rubber is predominant it has not time to participate in rapid movements in the manner implied by its elasticity, the consequence being a rapid and strong impact effect.
Spike designs known heretofore have not eliminated this visco-elastic impact, nor the road attrition effect resulting therefrom. Traditionally, the spike has obtained support from the rubber on the whole region of the flange part and by mediation of the respective surfaces of the body part, whereby the impact acting on the rubber has been powerful, causing road attrition and damage to the tire rubber.
Traditionally, the flow of rubber has not been taken into account when designing anti-slip means, as a result of which the body installed in the rubber will damage the rubber material, causing loosening of the spike and, directly or indirectly, damage to the body structure of the tire. Most usually such damage has been due to the circumstance that the shapes of the spike include sharp edges or detrimental discontinuities which cause wear of the rubber bonds as the rubber material flows past them and is urged against the sharp edges, due to the state of stress. Such discontinuities are also detrimental at which closed air pockets are formed. When the spike is pushed into the tire and rubber flows into such air pockets the pressure of the rubber is significantly higher than the air pressure and therefore the air is compressed in rapid impact; it is powerfully heated and causes damage to the surrounding rubber.